The present invention relates to gear transmissions, and more particularly to a system for decreased deflection of transmission housing walls that are subjected to bearing loads.
Fix-axes gear transmissions are used in many fields of technology, for instance in vehicles in manually shifted or automated form. They comprise a number of gearwheels that are located on a number of shafts. The shafts are supported by bearings that are seated in a transmission housing. The rotational axes of the shafts are thereby substantially fixed, hence the designation fix-axes gear transmissions. When torque is transferred from one gearwheel to another, significant gear mesh forces arise. These gear mesh forces will create reaction forces in the bearings. Thereby, in operation the walls of the transmission housings will be subjected to forces that will tend to deflect the walls. For proper function and life of the gearwheels and bearings, it is important that these deflections are limited.
In order to reduce weight, there is a trend towards using light alloys rather than iron based alloys for the transmission housings. Due to lower inherent stiffness of conventional light alloys, the deflections of the housing walls will increase. Furthermore, cost-efficient casting methods for light alloys do not permit as large wall thickness as is common in cast iron housings. That will further increase the deflections in light-alloy housings. Finally, the significantly larger thermal expansion of light alloys compared to steel will cause increased axial clearance in the bearings at operating temperature, which will give reduced bearing life.
There are some known ways to reduce the increased deflections of the walls of transmission housings made of light alloys. Firstly, reinforcing ribs will increase the stiffness of the walls while maintaining a small wall thickness. This can be seen in DE-10027375 and DE-10316321. That will, however, in general not give stiffness equal to that of a wall of full thickness unless the ribs are very large, which implies increased length of the transmission. Another way is to have a curved or bell-like shape of the wall instead of a more conventional flat wall. That will, unfortunately, also result in increased transmission length.
It is desirable to decrease deflection in the walls of a transmission housing.
So, there is a need for a way to increase the stiffness of fix-axes gear transmission housing walls but without the disadvantages regarding length, weight, and casting ability of prior art. According to an aspect of the present invention, a design is provided with a number of substantially straight rods connected between the housing walls where the bearings that support the shafts are seated. Thereby, the axial deflections of these housing walls will be reduced significantly.
Moreover, the corresponding misalignment of the bearing seats will be reduced, too. That will increase the bearing life.
In a preferred embodiment, the housing is made of light alloy. The rods will then compensate for the lower inherent stiffness (modulus of elasticity) of the light alloy.
In a further preferred embodiment, the rods are made of steel or any other material with lower thermal expansion than the housing material. Then, it is possible to counteract the thermal expansion, from ambient to operating temperature, of the housing relative to the shafts.
In yet a further preferred embodiment, the rods are preloaded at the assembly. When made of steel (or any other material that has a lower thermal expansion than the material of the housings), the rods will then also at temperatures lower than room temperature be able to reduce the axial deflection of the housing walls.
In an additional preferred embodiment, rods are located radially close to the intersections, when seen in axial direction, of the diametrically largest rotating parts of the shafts. Thereby, the rods will be close to the parts of the housing walls that would have the largest deflections if there were no rods. That will give a large reduction of the deflections.
In another preferred embodiment of the invention, the rods are screws that have a head in one end and threads in the other end. That will facilitate the assembly, especially if the rods are to be preloaded. Furthermore, if one of the housing walls to be connected by the rods is designed to have lubricating liquid on one side only, the threaded end of the screws can be mounted in a tapered boss in that wall. Thereby, possible risks for lubricating liquid leakage can be eliminated.
In still another preferred embodiment, the transmission is at least a part of a vehicle transmission for a heavy road vehicle, such as a truck or a bus. In such a transmission, the loads are very high. Thus, rods according to the invention will be of great advantage, especially if the transmission is the main section of a compound (or range) type vehicle transmission, for instance as shown in FIGS. 1, 3 and 5 in EP-1476681. There, rods as long screws can be mounted with the head against the housing wall between the main section and the compound (or range) section. There will be lubricating liquid on both sides of that wall. Thus, a possible lubricating liquid leakage around the screw heads would be irrelevant. The other end of the screws can then be mounted in tapered bosses in the opposite housing wall that faces the clutch. This will prevent leakage of lubricating liquid.
Finally, in a preferred embodiment, at least one rod is mounted inside a hollow shaft. Thereby, the rod will act very close to the bearings that support the hollow shaft.